Mitochondrial endogenous oxidative damage has been overestimated

The oxidatively induced DNA lesion 8-oxo-dG in mitochondrial DNA (mtDNA) is commonly used as a marker for oxidative damage to mitochondria, which in t urn is thought to be a fundamental cause of aging. For years, mitochondrial levels of 8-oxo-dG were believed to be similar to 10-fold higher in mtDNA than in nuclear DNA even in normal, young animals. However, studies in our own and other laboratories have shown that this lesion is efficiently repai red. Also, mutational consequences specific to 8-oxo-dG (G to T transversio ns) are rarely reported, In the present study, we showed that the levels of damage measured using high-pressure liquid chromatography/electrochemical detection and an enzymatic/Southern blot assay were comparable. The latter assay does not require isolation of mitochondria, and so this assay was the n used to determine the level of in vivo damage present in rat Liver mtDNA both with and without organelle isolation. Levels of 8-oxo-dG are approxima tely threefold higher when measured in mtDNA purified from isolated mitocho ndria than when measured without prior mitochondrial isolation. Furthermore , most genomes were free of endogenous enzyme-sensitive sites (i.e., they d id not contain 8-oxo-dG), and only after mitochondrial isolation were level s higher in mtDNA than in a nuclear sequence.